1. Field of the Invention
The present invention relates to a binder and an electrode for lithium batteries, and a lithium battery using the same, and more particularly, to a lithium battery binder and a lithium battery electrode with enhanced charge/discharge characteristics and lifespan characteristics, and a lithium battery containing the same.
2. Description of the Related Art
With the increasing supply of portable electronic devices, such as PDAs, mobile phones and laptop computers, their use range has been widened. Accordingly, the requirement for more compact, thinner, and lightweight batteries with high performance as power sources has been increasing, and much research on batteries has been conducted.
Since lithium batteries are lightweight and have higher energy density, they have been used as major power sources for such potable devices.
Cathode active materials for lithium batteries may include Li-containing transition metal oxides, such as LiCoO2 and LiNiO2, and chalcogen compounds, such as MoS2. Since these compounds have layer-crystalline structures, Li ions can be reversibly intercalated or deintercalated. Accordingly, these compounds have been widely utilized as cathode active materials for lithium batteries.
Metal lithium can be used as an anode active material. However, lithium ions of lithium are intercalated and deintercalated. Then, needle-shaped lithium dendrites grow on the surface of lithium because the lithium repeatedly dissolves and precipitates during charging/discharging of the battery. The needle-shaped dendrites have lower charge/discharge efficiency and cause an internal short-circuit by contacting a cathode.
To solve these problems, use of lithium alloy, metal powder, graphitic or carbonaceous materials, metal oxides, or metal sulfides, which can reversibly intercalates and deintercalates Li ions, as an anode material is under consideration. However, when a sheet-type anode made of a lithium alloy is used in a battery, the sheet-type alloy becomes thinner during charging/discharging, thereby degrading a current collecting property. Thus, the charge/discharge characteristics deteriorate.
When a sheet-type electrode is made of metal powder, a carbonaceous material, metal oxide, or metal sulfide powder, a binder is further used because these materials alone in powder form cannot form electrodes. For example, when manufacturing an anode using a carbonaceous material, it is common to add an elastic rubber-based polymer material as a binder.
When manufacturing an anode using metal oxides or metal sulfides, a conducting agent, in addition to the binder, is added to improve the charge/discharge characteristics. In general, when manufacturing an anode using a carbonaceous material, the carbonaceous material is pulverized into powder and a binder is added. However, if a conventional rubber-based polymer material is utilized as a binder, graphite particles may be coated depending on the amount of the binder, thereby hindering intercalation and deintercalation of lithium ions and deteriorating the high efficiency discharge characteristics.
If a conventional binder is used alone, regardless of the kind and form of a carbonaceous material, a large amount of binder should be added because a binding force between a metallic core material and the conventional binder is weak. However, when a large amount of binder is added to enhance the binding force, the surface of a carbonaceous material is coated by the binder. Therefore, the high efficiency discharge characteristics deteriorate. On the contrary, if a small amount of binder is used to maintain the discharge characteristics, the sheet-type electrode cannot be easily manufactured because a material for an electrode plate separates from the core material. Furthermore, the failure ratio increases in the manufacture of electrode plates.
Meanwhile, much research into polyvinylidenefluoride (hereinafter referred to as PVDF), which is commonly used as a lithium battery binder, has been carried out to improve its properties. For example, U.S. Pat. No. 6,426,165 discloses that the compounds with higher melting points have superior charge/discharge characteristics, wherein the melting points of PVDF polymers were measured using a differential scanning calorimeter (DSC). In this case, however, an anode surface and a cathode surface are separated from a PVDF binder layer. A mixture of PVDF, an anode material and a cathode material is not used to manufacture a battery. Therefore, the composition of the crystalline phases of PVDF, the anode active material, and the cathode active material in the battery, and how the composition varies depending on processing conditions are unknown.
In U.S. Pat. No. 5,246,796, the ratios of specific peaks in an X-ray pattern and charge/discharge maintenance rates are measured when a PVDF binder is dried at different temperatures. However, this patent simply shows that a higher drying temperature leads to lower charge/discharge efficiency. In addition, in this patent, the charge/discharge efficiency is low so that the charge/discharge maintenance rate is relatively low.
In U.S. Pat. No. 3,931,446, specific conditions in which α- and β-phases-containing films can be formed are defined. However, the relationship between the composition of a PVDF binder, the lifespan of a battery, and the battery efficiency is not disclosed in the patent.